Description of the Guayule. 53 



infrequently before the third year, and the amount of growth then does not more than 

 equal that of a single- stemmed retono in one year. At the end of three years the 

 retono makes a considerable plant (6 in the same plate), and flowers richly. The 

 influence which retonos would have in reforesting processes, both by their own growth 

 and by seedlings, can therefore be well appreciated, and probably with difficulty over- 

 estimated. Some basis for judgment in this regard will reward a study of the accom- 

 anying photograph l (plate 9, fig. B, in which the horizontal lines are to be regarded 

 as 10 cm. apart). 



From his comparative morphological and anatomical studies on " nor- 

 mal" parts and those of individuals ("rejets") arising from root buds, 

 Dubard (1903) draws the following conclusions: 



En re'sume', la multiplication par bourgeons radicaux est un fait peu normal 

 dans le regne ve"ge"tal ; elle donne naissance a des rejets d'organisation infeYieure, dans 

 la plupart des cas; chez quelques e"speces elle tend a s'e'tablir d'une facon re"guliere, 

 mais ne devient qu'exceptionnellement une sauvegarde effective de 1'espece. 



The inferior organization of the retonos studied by Dubard is always 

 in the direction of an anterior form "by virtue of hereditary ante- 

 cedents": "les rejets radicaux des diverse e"speces d'un meime genre 

 manifestent une convergence qui ne peut etre fortuite." 



The retonos of the guayule are in the same case. The absence of 

 medullary and of cortical canals is a marked return to a more simple 

 structure, as is also the absence of medullary stereome, in which, and in 

 the absence of canals in the medulla, we see an assumption of seedling 

 characters. But the retono assumes a still more ancient condition, we 

 may believe, in the loss of the cortical canals. 



Nevertheless, the guayule, while in this measure conforming to the 

 observations made by Dubard, can not on any account be relegated to 

 a subnormal category, characterized by comparative impotence in safe- 

 guarding the species. The frequently strong vegetative growth ; the early 

 maturation of flowers and seeds ; the already established root-system ; the 

 cincture of the mother root tending to separate the retono physiologically, 

 if not always structurally, from the parent plant (fig. n); its frequently 

 wide separation from this; its ability to gain a foothold where seedlings 

 must surely perish; all these facts heighten the importance of the retono, 

 despite the relatively small numbers in which they are found, in enabling 

 the species to maintain a foothold. It seems, indeed, not unlikely that 

 a further classification beyond that of Dubard will be necessary one for 

 those plants in which the retono is of great importance in this regard. 



A comparison at this point between the guayule and the mariola is 

 of special interest, because, while they are closely related species, their 

 methods of vegetative reproduction are quite distinct. 



In the first place, the root-system in the mariola differs in that the laterals 

 run at a steeper angle into the soil. Occasionally retonos are formed, but, as far 

 as my observation goes, always close to the plant, within, say, 5 cm. What always 

 happens, however, is this: From the basal portion of the stem, where there are 

 many dormant buds, as a sequence of the short internodes marking the slow initial 

 growth of the seedling, new, slender shoots arise, growing to a height of 30 cm., 

 more or less, in two months. From the base of each such shoot an adventitious 

 root starts out, immediately above the point of origin of the shoot. This usually 

 single root develops as a tap-root, and supplies all the water for the daughter shoot, 



1 F. E. Lloyd, 1908^. 



